Financial stocks and energy shares, two sectors that could face increased regulation after President Barack Obama's re-election, were the weakest on the day. The S&P financial index .GSPF lost 3.5 percent, while the S&P energy index .GSPE fell 3.1 percent. An S&P index of technology shares .GSPT slid 2.8 percent as the stock of Apple Inc (AAPL.O) entered bear market territory.
Obama's victory had been anticipated, though many polls indicated a close race between the president and Mitt Romney, his Republican challenger, going into election day.
The election was considered a major source of uncertainty for the market, but now the focus turns to the fiscal cliff, with investors worrying that if no deal is reached over some $600 billion in spending cuts and tax increases due to kick in early next year, it could derail the economic recovery.
The Republican Party retained control of the U.S. House of Representatives, while the Senate remained under Democratic control.
David Joy, chief market strategist at Ameriprise Financial in Boston, said this kind of divided government was disappointing "since that configuration has resulted in gridlock and there's no clear path towards unlocking that.
"It holds implications for how quickly we resolve the fiscal cliff issue, or whether it gets resolved at all," said Joy, who helps oversee $571 billion in assets.
The market's losses were broad, with pessimism exacerbated by overseas concerns after the European Commission said the region would barely grow next year, dashing hopes for improvement in the short term.
Still, some viewed the day's slide as a buying opportunity, saying it was unlikely that no deal would be reached on the fiscal cliff and arguing that Europe's troubles were already priced into markets.
"There's no question that Europe is lagging the rest of the developed and emerging world, but stocks will find a base soon, when investors start seeing through some of the smoke over the region and cliff," said Richard Weiss, who helps oversee about $120 billion in assets as a senior money manager at American Century Investments in Mountain View, California.
The Dow Jones industrial average finance/markets/index?symbol=us%21dji">.DJI slid 312.95 points, or 2.36 percent, to close at 12,932.73. The Standard & Poor's 500 Index .SPX fell 33.86 points, or 2.37 percent, to 1,394.53. The Nasdaq Composite Index .IXIC lost 74.64 points, or 2.48 percent, to close at 2,937.29.
The S&P 500 closed below the key 1,400 level for the first time since August 30, while the Dow ended under 13,000 for the first time since August 2.
About 7.81 billion shares traded on the New York Stock Exchange, the American Stock Exchange and Nasdaq, slightly below last year's daily average of 7.84 billion, though Wednesday's volume did surpass that of many recent sessions.
Contributing to the Nasdaq's decline, Apple shares fell 3.8 percent to $558, off 20.8 percent from an all-time intraday high of $705.07 set on September 21. That slump puts the stock of the world's most valuable publicly traded company in bear market territory.
Despite Wednesday's sell-off, all three major U.S. stock indexes were still up for the year. At Wednesday's close, the Dow was up 5.9 percent for 2012 so far, while the S&P 500 was up 10.9 percent and the Nasdaq was up 12.8 percent.
Wednesday's plunge was a reversal from Tuesday's rally when voting was under way. Defense and energy shares were among the market leaders that day, causing speculation that some investors were betting on a Romney win.
On Wednesday, an index of defense shares .DFX fell 2.9 percent, its biggest one-day drop in a year. Shares of United Technologies (UTX.N) dropped 2.9 percent to $77.68 while Lockheed Martin (LMT.N) sank 3.9 percent to $91.15.
Energy shares fell as investors bet that the industry may see increased regulation in Obama's second term, with less access to federal lands and water. Crude oil shed more than 4 percent while an index of coal companies .DJUSCL plunged 8.8 percent. Coal firms Peabody Energy (BTU.N) lost 9.6 percent to $26.24 and Arch Coal (ACI.N) sank 12.5 percent to $7.58.
Among financials, JPMorgan Chase & Co (JPM.N) fell 5.6 percent to $40.46 and Goldman Sachs (GS.N) dropped 6.6 percent to $117.98.
"The notion that you may have gotten a respite on the financial services side (with regulation) if Romney had been elected is obviously being unwound," said Mike Ryan, chief investment strategist at UBS Wealth Management Americas in New York.
Healthcare stocks were mixed as President Obama's re-election rules out the possibility of a wholesale repeal of his healthcare reform law, though questions remain as to what parts of the domestic policy will be implemented. The S&P health care index .GSPA shed 1.9 percent. In contrast, Tenet Healthcare (THC.N) was the S&P 500's biggest percentage gainer, up 9.6 percent at $27.34.
In 2008, stocks also rallied on election day, but then fell by the largest margin on record for a day following the vote, with each of the three major U.S. stock indexes posting losses ranging from 5 percent to 5.5 percent.
After the bell, both Qualcomm Inc (QCOM.O) and Whole Foods Market Inc (WFM.O) reported results. Qualcom's revenue beat expectations, sending shares up 8 percent to $62.75 in extended trading, while Whole Foods dropped 3.3 percent to $92.75 after the bell. In the regular session, Qualcomm slid 3.7 percent to close at $58.12, while Whole Foods dropped 2.1 percent to $95.93.
